feed drupal 8 custom block with json decode content

i have created a block and try to feed it with WS request

function amu_news_preprocess_block(&$variables)
 {

  if ($variables['elements']['#configuration']['id'] == 'amu_news_block') {
    $client = new GuzzleHttpClient();     
    $url = 'mywebserviceretrievingjson.org';
    $response = $client->request('GET', $url);

    //this display json:
    $content = $response->getBody()->getContents();
    // this generate server error although $content contains array of nodes as expected:
    //    $content=json_decode($response->getBody()->getContents(),true);


    $variables['elements']['content'] = $content;
    //why do i need to do that next line ? i dunno but this makes json content display on my twig template:
    $variables['content'] = $variables['elements']['content'];
}
}

i am trying to display feeds as array or object